Prevent white space in the input not working

You should use validation for this. Probably something like

<input type="text" v-model:value="username" pattern="[A-Za-z0-9]+">

Also you can do following:

<input type="text" v-model:value="username" pattern="[A-Za-z0-9]+" onkeydown="if(['Space'].includes(arguments[0].code)){return false;}">

I've already encountered issues when using patterns, but I believe it was when matching a pattern with the number type and only on some mobile devices. In your case, it shouldn't be an issue imo.

If it is (or becomes) one, you can simply handle input suppression with Vue using v-model and a watcher. After the v-model sets the data property, the watcher is triggered and cleanup can be made (all this process is not visible to the end user, it feels the same as a keydown suppression).

watch: {
  username(value) {
    this.username = value.replace(/ +/g, '');
  }
}

You can complexify the regex pattern to reach the desired validation.

thanks ::)))

JavaScript to allow numbers only. It will NOT allow for spaces.

    <SCRIPT TYPE="text/javascript">
        <!--
        function numbersonly(myfield, e, dec)
        {
            var key;
            var keychar;
            if (window.event)
                key = window.event.keyCode;
            else if (e)
                key = e.which;
            else
                return true;
            keychar = String.fromCharCode(key);
            // control keys
            if ((key==null) || (key==0) || (key==8) ||
                (key==9) || (key==13) || (key==27) )
                return true;
            // numbers
            else if ((("0123456789").indexOf(keychar) > -1))
                return true;
            // decimal point jump
            else if (dec && (keychar == "."))
            {
                myfield.form.elements[dec].focus();
                return false;
            }
            else
                return false;
        }
        //-->
    </SCRIPT>

On the field

onkeypress="return restrictInput(this, event, integerCommaOnly);"